This invention relates to electrographic development and more particularly to an improved method of plural stage development. While the following disclosure refers to a preferred embodiment employing liquid development, it will be understood that the invention is equally applicable to systems using dry developers.
In the liquid development of electrostatic charge latent images, as in electrophotography and in other processes that form and develop electrostatic charge patterns, a substrate having a charge pattern on its surface is contacted with a liquid developer which is essentially a suspension of colloidal toner particles in an insulating liquid. Liquid developers normally contain also a stabilizer or charge control agent. Liquid developers can be used in single stage or plural stage development processes. Examples of the latter may include the sequential development on a photoconductor of two or more color-separation images, the annotation of a previously developed image, or the repeated re-exposure and developemnt of images on a reusable photoconductor, with transfer of images after each development.
Some plural stage development processes which use liquid developers have been found to be especially significant for the electrophotographic reproduction of multi-color images of graphic arts quality. In these processes electrostatic latent images are formed sequentially on a chargeable substrate such as an electrophotographic medium or photoconductor, with liquid development or toning of each latent image before the next is formed. An example of this kind of process involves exposing an electrophotographic medium sequentially through a series of four registered color-separation transparencies wtih four sequential development or toning stages.
Heretofore it was thought that the use of a low optical density p-type photoconductor was impractical for the formation of such high quality multi-stage or multi-color images. This was because such photoconductors generate mobile "holes" and trapped electrons within the thickness of the photoconductor as compared to other types of photoconductors, wherein principally mobile charges are produced. This belief resulted, at least in part, from the fact that erase methods used for such photoconductors were unable to eliminate the electrons trapped throughout the thickness or bulk of the photoconductor, or near the positive surface, because of the inability of the trapped electrons to move. That problem is exaggerated in an imaging system requiring the use of a toner-carrying, transfer-aiding overcoat which may be used to transfer the applied toners to a final receiver sheet after multi-stage or multi-color development of the photoconductor. This is particularly true when the overcoat is of a nonconductive nature.
It has been found that nonconductive overcoats are necessary when half-tone images are generated, as is required in the graphic imaging field. A conductive overcoat has been found to be unsatisfactory for generating half-tone images because the half-tone dots on a conductive surface tend to undergo image spread and will effectively disappear under adverse humidity conditions. Even continuous tone images have been found to severely degrade under such conditions using conductive overcoats.
Thus, the use of a nonconductive overcoat with a p-type photoconductor for multi-stage imaging has exacerbated the problem of trapped charges within the thickness of the photoconductor. The build-up of trapped charges within the photoconductor under such conditions exhibits itself as a residual charge that builds with each successive charge and expose cycle. The net effect is an ever increasing electrical toe voltage (toe rise) in the photoconductor. That is, the film voltage that is achieved as a result of large exposures increases with accumulated charge-expose cycles. This is illustrated in FIG. 1, wherein the effect of large exposure versus voltage is illustrated for four successive charge-expose cycles. It will be seen that in the initial charge-expose cycle, the voltage achieved with high exposures is the lowest value, whereas with each successive charge-expose cycle, the same amount of exposure results in ever increasing final voltages. As a result, in order to maintain a usable voltage range over which the photoconductive film can operate, the initial charging voltage must be increased with each cycle. It has been found that cumulative increases in the film toe voltage of as much as 100 volts or more can occur, which results in the need to increase the initial charging voltage by an equivalent amount. Not only does this complicate the control of the charging apparatus, but it can quickly exceed the equipment and film capabilities so that the process can no longer provide the desired voltage differential. Moreover, it has been found that the toe rise is unpredictably variable so that it is difficult, if not impossible, to control the entire process to provide satisfactory film performance. If provisions are not made to maintain the useable voltage range in systems utilizing a photoconductor having the foregoing phenomena of toe voltage rise with successive charge-expose cycles, the resulting images will have an undesirable susceptibility to image variation with variations in work place humidity. A variation in image quality across the area of the image can also occur under such operating conditions. More importantly, it has been found that with increases in the toe voltage noted above, it is impossible to achieve high image density with succeeding stages or colors. All of these factors are detrimental to obtaining graphic arts quality images.
The elctrophotographic method in which the present invention operates generally comprises uniformly charging a photoconductive element, exposing the photoconductive element to a pattern of actinic radiation to form a latent electrostatic image, developing the latent image with, for example, a liquid developer composition comprising a carrier liquid, a toner and charge control agent, rinsing the developed surface of the photoconductive element with a rinse solution, and drying the image. Thereafter, the surface of the photoconductive element is recharged and exposed to a pattern of actinic radiation to form a second latent electrostatic image which is developed with a liquid developer.
The method of the present invention is useful in any electrostatic imaging process wherein a charge pattern is formed and developed with a developer on a surface which has previously been developed. It is particularly useful, however, in combination with a recently developed electrophotographic method of making lithographic color proofs. This new method and a photoconductor for use therein is described in the copending U.S. patent application of Ng et al., Ser. No. 773,528 filed Sept. 6, 1985, now U.S. Pat. No. 4,600,669. In this method a photoconductor which has a uniformly charged thin transparent dielectric overcoat is subjected to a series of exposures through registered color separation transparencies. After each exposure the dielectric layer is developed with a liquid developer, the surface is again uniformly charged and exposed. The sequence is repeated for each of the color transparencies, usually four, with all of the developed images being superposed to form a multi-color image on the overcoat.